The An-225 flies on-demand charter cargo services and is owned by a company called Antonov Heavy Transport. There are no scheduled routes as far as I know. It has most recently been spotted in Luxembourg, and in the U.S.: Cleveland, Ohio; Orlando, Florida; and Houston, Texas.

When it came to Cleveland, it was carrying a shipment of large custom-made ball bearings for a local factory, and quite a bit of scientific glassware.

The An-225 was designed to carrying the then-Soviet Union's space shuttle Buryan, but the Soviet space program soon ran out of money for space shuttle missions and the Buryan was mothballed after only one flight. There are a couple photos of the An-225 carrying the Buryan in the A.net database.

I was actually lucky enough to see it take off from Houston Intercontinental Airport. I was dropping some family off and went around waiting for them at the terminal, and the runway came into my field of view right as it was lifting off. Amazing.